IS 12208: Method for measurement of earth pressure by hydraulic pressure cell

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IS 12208 (1987) : Method for measurement of earth pressure
by hydraulic pressure cell [CED 43: Soil and Foundation
Engineering]

Jawaharlal Nehru
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IS 1 12208 . 1987

( Reaffirmed 2006 )

Indian Standard

METHOD FOR MEASUREMENT OF EARTH
PRESSURE BY HYDRAULIC PRESSURE CELL

( First Reprint SEPTEMBER 1998 )

UDG 624-131 -386 : 627-824

® Cofcmght 1988

BUREAU OF INDIAN STANDARDS

MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI 110002

Gr 2 7«w 1988

IS s 12208 - 1987

Indian Standard

METHOD FOR MEASUREMENT OF EARTH
PRESSURE BY HYDRAULIC PRESSURE CELL

0. FOREWORD

0.1 This Indian Standard was adopted by the
Bureau of Indian Standards on 26 August 1987,
after the draft finalized by the Foundation
Engineering Sectional Committee had been
approved by the Civil Engineering Division
Council.

0.2 Total pressure at the foundation of earth fill
dams and embankments as well as earth pressure
on retaining walls are required to be monitored
to evaluate their post construction behaviour and
taking timely remedial measures for the struc-
tures showing distress.

0.3 Different types of earth pressure cells are used
for the measurement of such in-situ stress. They
are generally of two types:

a) Flexible diaphragm type, and

b) Stiff cylinder type.

The flexible diaphragm type cell consist of a
flexible circular or rectangular diaphragm attached
to a rigid stiff case where the pressure is measured
due to continuous displaced shape of the flexible
diaphragm, whereas in the second type, the axial
compression of the stiff prismatic element, usually
enclosed within a case to isolate it from the lateral
stresses of the surrounding soil mass is used to
sense the total pressure.

0.4 Some of the other systems adopted for mea-
suring earth pressure are:

i) Electrical resistance strain gauge,

ii) Semiconductor strain gauge,

iii) Vibrating wire system,

iv) Closed fluid system ( usually called Gloetz
or hydraulic pressure cell ), and

v) Pneumatic system where air pressure is
used to balance the stress on the cell.

0.5 Out of the above systems, the strain gauge
type, vibrating wire type, and the closed fluid
system, that is, the hydraulic pressure cell are the
most accurate and are most commonly used.
Resistance strain gauge types are easy to use and
have linear rapid response. But they are susceptible
to damage and are affected by the moisture of
the fill material in long use. Vibrating wire type
are more durable but have non-linear response.
More rugged and durable are closed fluid systems
or hydraulic pressure cells which are universally
adopted for measuring the earth pressure. This
Indian Standard covers the use of such type of
pressure ceil.

0.6 For the purpose of deciding whether a parti-
cular requirement of this standard is complied
with, the final value, observed or calculated, ex-
pressing the result of a test or analysis, shall be
rounded off in accordance with IS : 2-1960*. The
number ot significant places retained in the roun-
ded off value should be the same as that of the
specified value in this standard.

♦Rules for rounding ofF numerical values ( rtvised ).

1. SCOPE

1.1 This code deals with measuring the total
pressure in earthfills, dams, embankment as well
as pressures on the surface of retaining walls,
bridge abutments, etc, by using the technique of
balancing the fluid pressure in the cell by a pre-
ssure applied to the reverse side of the transducer
diaphragm. The same method may also be utilized
with slight modification for measuring the magni-
tude of stresses on tunnel linings rock and concrete
masses and other structures inclusive of hydraulic
ones.

2. TERMINOLOGY

2.0 For the purpose of this standard, the follow-
ing definitions shall apply.

2.1 Total Pressure — Total pressure at a given
location is the sum of effective soil pressure and
pressure due to ground water or air pressure.

2.2 Pressure Change — Pressure change is the
difference between total pressure at any given
time and the total pressure value at the time of
cell installation.

2.3 Balancing Pressure — It is the pressure
applied to the reverse side of the transducer
diaphragm to balance the fluid pressure built up
in the celk

2.4 Cell Fluid — Hydraulic fluid used for filling
the cell.

1

IS : 12208 - 1987

2.5 Measuring Fluid — Hydraulic fluid used in
transducer and read out unit,

3. EQUIPMENT

3.1 It consists of (a) pressure cell, (b) fluid reser-
voir, (c) a pump with pressure gauge to measure
the applied pressure, and (d) a detector to indicate
the fluid return from the cell.

3.1.1 The pressure sensor consists of a flat jack
of suitable size filled with hydraulic fluid and
connected to the pressure measuring transducer.
The flat jack is usually made from two flat sheets
of steel, welded around the periphery. They can
be circular or rectangular in plan, its size depen-
ding upon the measuring location. The choice of
cell fluid depends on actual requirements. Liquid
of low viscosity is usually used for higher pressure
and also for long delivery lines.

3.1.2 The pump should be capable of applying
a pressure at least 20 percent in excess of the
maximum pressure to be measured. It should be
able to increase/decrease the pressure gradually
and to hold up the pressure for several minutes as
per the requirements,

■^.l.S The return flow indicator system should
be capable to detect a flow of less than lU percent
of the maximum flow possible through the cell
transducer and tubing and a measuring accuracy
better than ± 2 percent of the measured pressure
throughout the range,

4. SELECTION OF PRESSURE CELL

4.1 The stiffness of the cell should be similar to
that of the material in which it is to be embedded
to ensure smooth stress transfer. The gap between
the plates forming the cell should not exceed
1 mm, and the ratio of the cell diameter y side
length to the cell thickness should be greater than
20: 1.

4.2 The material of the cell, transducer, and all
ancillary components, should be so selected as to
resist corrosion due to surrounding materials,
ground water, cell fluid and measuring fluid.

4.3 The design and material of the transducer
should be such as to ensure minimum diaphragm
inertia so that the pressure in the measuring fluid
corresponds closely with that in the cell fluid at
the time of balancing.

5. CALIBRATION OF EQUIPMENT

5.1 The cell should be checked in a compression
testing machine for its range, to determine the
edge-effects and to evaluate proper correction
factor. The equipment needs to be calibrated for
temperature effects at the cell location.

5.2 The complete assembly should be checked to
determine diaphragm inertia and the effects of
delay between pumping and fluid return.

5.3 The read out pressure gauges should be
calibrated using a standard dead weight pressure
gauge tester.

6. ACCURACY

6.1 Overall accuracy requirements should be spe-
cified. Generally, it should be better than ± 5
percent of the pressure to be measured, inclusive
of the combined effects of inaccuracies due to lag,
temperatures, tube pressure losses and gauge cali-
bration errors.

7. INSTALLATION

7*1 Selection of Locations

7.1.1 Ceils are generally installed in pairs or
clusters to measure pressure in different directions
at the same location. Adjacent cells should be
separated by a distance of at least 1 cell diameter
in such a way as to prevent the presence of a cell
affecting readings on adjacent cells.

7.1.2 The distribution of the cells should be
such that each ceJl should represent a particular
type of material.

7.1.3 The cell should be in complete contact
with the surrounding material which should not
have any protrusions or non-imiform material
that may result in stress irregularities on the cell.

7.1.4 The cells should not be installed in
locations exposed to appreciable tcmperatiure
changes due to exposure to direct sunlight or cold
wind. They might have to be insulated in such
cases.

7.2 Installation in Soils

7.2.1 When the cells are to be installed in natu-
ral soil or fill embankment, an overall excavation
of stabled slopes and of sufficient dimensions is to
be made to accommodate the cell cluster. Then
individual pockets, each being of size approxi-
mately twice that of the cell to be installed, are
hand dug at the correct locations and at correct
inclination taking utmost care not to disturb the
surrounding soil strata.

7.2.2 Rock fragments greater than 1/10 cell
diameter or size, except in cases of rock fill em-
bankments, arc to be removed and replaced by
fine grained material, and compacted into the
voids. The cells are then fixed in position, taking
care to see that they are fully in contact with the
underlying material. Each pocket is then back
filled with fine grained material, hand compacted
to a density similar to that of the original fill,

7.2.3 In case of rock fill embankment, the
pocket should be larger and the pocket back fill
should be graded, with the finer material placed
adjacent to the cell and the coarser adjacent to
the embankment.

IS 1 1220S - 1987

7.2.^ The cluster excavation is then back filled
and compacted with natural embankment mate-
ria], having removed the big rock fragments.
Three layers of 10;20 cm each, should first be
placed and hand compacted before completing
the back fill with light mechanical equipment.
No heavy vibratoiy rollers should be used until at
least 2 m of fill has been placed on the cells
through controlled compaction.

7.3 Installation at the Interface Between
Soil and Concrete or Rock

7,3.1 When placing cell adjacent to piers, piles,
retaining walls, culverts and other structures, the
cells may either be attached to the form and
placed in the structure, or fastened to the struc-
tures, prior to the back tilling, or embedded in
the back fill, a short distance away from the
structure. The contact between the cell and the
back fill material should be effected by means of
a layer of fine grained material as indicated
in 7.2.2.

8. CONNECTING, FILLING AND CHECK-
ING THE CELLS

8.1 Cosuiection

8.1.1 The tcrmiral equipment is to be fixed
securely, either nearer to the cells on a wall or
remote from them in an instrument house. The
terminal panels should be precheckcd for proper
functioning of the valves and for leakage in the
system.

8.1.2 The labelled tubing is to be connected
to the appropriate terminals and secured in place.
A test should be made on each cell while accessi-
ble, fo^ repair and replacement to ensure proper
functioning of the completed hydraulic system.

8.2 Filling

8.2.1 The cells, with liquid as measuring
medium, may be filled with the measuring fluid
by the read out unit pump but usually, it is more
convenient to fill them by gravity from a fluid
reservoit. While filling the system, care should be
taken to see that the delivery tubes arc also com-
pletely filled up. Bleed points should be provided
at positions where air entrapment is likely.

93 Checking

8.3.1 Each connecting tube in the systems
should be temporarily disconnected from the cell
and the complete system tested to a pressure of
at least 120 percent of the anticipated ma^cimum
use, reconnected, refilled and checked for leakage
along all the tube lengths.

8.3.2 After installation of the cells and back
filling, the cell pressures should again be recorded
and back pressured to ensure a small positive
reading after compensation and for the fall in

pressure or any negative pressure developed
during installation.

9. PROCEDURE FOR TAKING READINGS

9.1 After proper calibration and functional che-
cking of the read out unit, it is connected to the
cell delivery and return tubes, taking care to
avoid entrapment of air in the delivery tube.

9.2 The supply pressure is increased gradually
until a return flow is recorded. The return flow
should be maintained for a period of at least
4 minutes to ensure removal of air bubbles and to
establish steady conditions. An approximate read-
ing of the delivery pressure is then noted.

9.3 The pressure is released and again increased
at a very slow and constant rate ( usually 3-4
cm'/min until return flow is observed, which is
noted. There is usually a characteristic peak in
the pressure flow curve due to the inertia of the
diaphragm valve. This should be ignored and the
steady pressure taken as the reading.

9.4 Further, readings are taken and recorded
until a consistent reading, /*,, is established which
would be the average of a minimum of three
readings.

9.5 Delivery line pressures should preferably be
maintained between the readings, at a level that
will avoid the entry of air, yet well below the long
term burst pressure of the tubing.

9.6 The procedure is repeated for all other cells
whose readings are required to be taken.

10. CALCULATIONS

a) The cell pressure, P, is estimated from the
reading, P,, by applying the corrections as
follows;

P = ( P, - P, - Pj - P, ) X £

where

Pi = pressure reading,

Pi B= initial cell pressure applied during
flrst installation, and /or other factors
subsequently adjusted by compens-
ation for shrinkage,

Pn «= static head correction for the pre-
ssure due to difference in between
the cell and read out unit ( liquid
only; for gas P^ — ),

Pf «= correction for frictional losses in the
fluid delivery line, and

E = multiplying factor ( less than 1 '0 ) to
compensate for edge effect of the
cell

b) In most applications, only changes in pre-
ssure are of interest. In most cases, an in-
itial readings, Pj, is taken after completion
of the installation and it includes the effect
of Pjj and Pf which remains constant for a

IS : 12208 • 1987

particular set of cells. In these cases,
P = ( P, - P, ) X £

c) The elevation correction, P^ may be cal-
culated as follows:

^h - r ( Ai - AO

where

r = unit weight of measuring fluid,
g/cm3( for gas, this unit weight
is zero ), and

hi — Aj = difference in elevation ( cm ) be-
tween read out unit and cell
( positive when the cell is below
the read out unit ) .

Pja, is then obtained in g/cm^ and can be
converted to Pa W multiplying by 0*098 1.

d) The tube friction correction Pf should be
measured during installation, before con-
necting the cell, and is the pressure required
to maintain a steady flow through the
tubing at a flow rate similar to that obtai-
ned during measurement. Under normal
conditions, with unobstructed and correctly
selected tubing, this correction should be
small.

e) The edge effect correction, E, should be
established on the basis of control tests in
a compression machine,

f) In addition, a temperature correction may
be required in some specialized applications.
The correction Ft to be subtracted from
the readings may be expressed as

where

{tf — ti) is the temperature increase ( ''G )
from the time of the initial reading Pi and
A*t is a coefficient expressing the response
of the system ( cell, fluid and surrounding
material ) to temperature. The actual value
of Ki will depend on the size of the cell.

11. REPORT

11.1 Result should be presented in two forms
of reports:

a) Installation report, detailing basic data on
the instrumentation system at the time of
installation, and

b) Monitoring report, submitted periodi-
cally, giving the results of routine obser-
vations.

Frequent monitoring report is very essential
to minimize delay between the detection of ad-
verse behaviour and the remedial measures that
may be necessary.

11.1.1 Installation Report — I nstal lation
report should include the following:

a) A description and diagram of the monitor-
ing equipment installed, including their
detailed performance, specifications and
manufacturer's literature;

b) A location plan showing details of the
pressure cell location, details of methods
used for installation, calibration and
monitoring;

c) A location plan showing details of the pre-
ssure cell locations with respect to the
structural configuration and the surround-
ing soil, rock or concrete conditions; and

d) For each cell, a report giving the initial
installation pressure and wherever applica-
ble, the pressure after compensation for
shrinkage. Details of calibrations and cor-
rection factors should be included, along-
with details of any problems, encountered
during installation of each cell.

11.1.2 Monitoring Reports — Monitoring,
Reports should include the following:

a) An up-to-date field data sheet with results-
and graphs;

b) A brief commentary, drawing attention to
significant pressure changes and all instru-
ment malfunctions occurring since the pre-
ceding report; and

c) The result of any calibration or checking of
instruments carried out since the preceding
report.

12. PRECAUTIONS

12.1 Edge effects occur due to the presence of the
weld around the circumference/periphery of the
cell. They are greatest when the cell is small and
rigidly constructed. The thickness of flange
around the cell periphery partially affects the
transfer of stresses to the cell from the surrounding
material. Edge effect is difficult to estimate but
may be determined experimentally by embedding
the cell in a large block and then subjecting it to-
uniaxial compressive stress under controlled lab-
oratory conditions.

12.2 The most reliable method for temperature
correction is to provide an additional cell that is^
exposed to the ambient temperature at the loca-
tion, but not subjected to any pressure. Any pre-
ssure increase/decrease noted in this control cell
due to temperature variation, may then be
deducted from the pressure indicated by the adja-
cent cells installed in the structure.

12.3 Various other sources of error are due to-
(a) inadequate matching of the cell and surroun-
ding material stiffness, (b) placing the cell at an.
unrepresentative location in the structure, and
(c) installation of an inadequate sized cell which
can be avoided by proper planning of the instru-
mentation programme before the start of the work.

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